# Poly(3-hexyltiophene-2,5-diyl) (P3HT) reactions with inorganic salts in aqueous solution

I would need for my research at least as a starting point to know what kind of reactions occur at the interface of P3HT and an electrolyte. The electrolyte is simple $\ce{KCl}$ solution, maybe with $\ce{HCl}$ or $\ce{KOH}$ added. The reactions of polytiophene with the same type of electrolyte will do, if there is no information on P3HT. The two molecules don't seem to be that much different to me from the point of view of electrolytic reactions.

What I really need is information about reaction kinetics, at least in the form of some reaction constants, but simple stochiometric equations are still better than nothing at all, so I'm interested in those, too. I have been searching for some time now, but I'm not a chemist, and I don't really know where to look to find something like this.

• Assuming the electrolyte is inert (e.g., won't oxidize or reduce the P3HT), then there are no likely reactions. I would look up electrochemical potentials of KCl and P3HT, but I don't expect much would happen. Feb 22 '16 at 21:11
• @GeoffHutchison If you measure the characteristics of a transistor made as such that you put two electrodes under a P3HT thin film, drop a drop of electrolyte on the top, and insert an electrode into the electrolyte, the characteristics of the transistor drastically change. These transistors are sensitive to both changing the pH or salt concentration. The sensitivity comes from capacitive moulation, but also there is a continuous drift in the characteristics, that might be due to surface reactions. My first impression also was that it doesn't react, but seemingly experiment shows otherwise. Feb 22 '16 at 21:55

I'll point to a very recent paper on oligothiophenes for solution-gated FET sensing that suggests there are no chemical reactions. (This article popped up in my feed - there are many others on solution-gating organic electronics.)

$\alpha,\omega$-dihexyl-sexithiophene thin films for solution-gated organic field-effect transistors. Appl. Phys. Lett. 108, 073301 (2016)

The authors look at many factors influencing the device performance:

Finally, excellent transistor stability is confirmed by continuously operating the device over a period of several days, which is a consequence of the low threshold voltage of DH6T-based SGOFETs. Altogether, our results demonstrate the feasibility of high performance and highly stable organic semiconductor devices for chemical or biochemical applications.

The devices in this article use KCl and phosphate buffer. They vary the pH and ionic strength.

I haven't seen anyone suggest chemical reactions, and I'm not sure how it would occur. Certainly the oligo- or polythiophene could degrade over time, but most articles argue these are stable for long-term use.

The authors do note a small drift on the time scale of days:

both changes may possibly be attributed to the passivation of trap states at the semiconductor-electrolyte interface by water molecules penetrating into the thin film

I can't speak to the drift you see, but I'd make sure that:

• You aren't seeing evaporation of solvent over time (e.g., from device heating) that's changing your electrolyte concentration.
• You use a proper reference electrode
• You try removing water via vacuum and repeat the experiment (i.e., you drive off any water penetrating the semiconductor film)

Certainly organic semiconductors are porous, and while mostly hydrophobic, it's not unreasonable to expect that after days of use, some water may migrate into the film.

• @GeoffHutchinson Thank you very much. To be honest, I'm not doing the experiments, but the simulations, and my experimentalist conterpart told me this one, and that it would be nice to have a model of the drift. I'll definitely talk with her, and she promised to show me how she fabricates and measures these devices. However, she is no amateur, and I don't suppose she would talk about a drift that comes from experimental error. What might happen however, is that the P3HT thin film itself degrades when operating the device (I read it in a Master's thesis though, so it's not 100% sure). Feb 23 '16 at 19:58
• @GeoffHutchinson I didn't specify it in the question, that is true, but there is a fundamental difference between the transistor that they have made and ours: ours is solution processed, thus the P3HT film is disordered, while in the article they have used molecular beam deposition which results in single crystals. I could also imagine that the disordered P3HT film is less stable, than a single crystal (although right now I don't know why). Maybe not because of surface reactions then, but some internal change. Can this be possible? Feb 23 '16 at 20:02
• @user3237992 - I'd be happy to consult more, but find my e-mail. Definitely if you're doing simulations, see how they're making the devices. I would not be surprised solution-deposited P3HT has differences - it's likely much more porous than the crystalline films. But I doubt it's from a chemical reaction - more likely some sort of water interpolation or rearrangement of the film. As I said, please send me an e-mail. Feb 23 '16 at 20:59